Thermosensitive apparatus for demonstrating heat phenomena



y 1960 A. STRICKLER 2,945,305

THERMCSENSITIVE APPARATUS FOR- DEMONSTRATING HEAT PHENOMENA Filed Aug. 3, 1953 5 Sheets-Sheet 1 //v VNTO/?. ALLEN STRICKLER BY HIS ATTORNEYS. HHPIPIS, K/ECH, RUSSELL 6: Knew July 19, 1960 A. STRICKLER HENOMENA THERMOSENSITIVE. APPARATUS FOR DEMONSTRATING HEAT P 5 Sheets-Sheet 2 Filed Aug. 3, 1953 mus/woe. ALLEN -5TR/CKLEI? BY H/S HTTORNEYS. K/ccH, RUSSELL 9: KERN July 19, 1960 A. STRICKLER THERMOSENSITIVE APPARATUS FOR DEMONSTRATING HEAT PHENOMENA Filed Aug; 3, 1953 5 Sheets-Sheet 3 INVENTOR- ALLEN STRICKLER BY, H/S ATTORNEYS. HARRIS, K1501. RUSSELL 2 KER/v v July 19, 1960 A. STRICKLER THERMOSENSITIVE APPARATUS FOR DEMONSTRATING HEAT PHENOMENA 5 Sheets-Sheqt 4 Filed Aug. 3, 1953 /NVE.NTO/?. ALLEN STRICKLER BY H/s ATTORNEYS; HARRIS, mac/1g Russ/5L1. 2 KER/v July 19, 1 960 A. STlCKLER 2,945,305,

THERMOSENSITIVE APPARATUS FOR DEMONSTRATING HEAT PHENOMENA' Filed Aug. 3; 195a Sjhgets-Sheet 5 i .2561." F923 I e24 .500 .550 460 540 620 700 Wave/any). in MIN/microns /N VNTO 1?. ALLEN 5TRICKLR BY HIS FTTORNEYS. HARE/s, KnscH, RUSSELL 6: KERN United States Patent fitice 2,945,305 Patented July 19, 1960 THERMOSENSITIVE APPARATUS FOR DEMON- STRATING HEAT PHENOIVIENA Allen Strickler, 620 N. Brighton St, Burbank, Calif. Filed Aug. '3, 1953, Ser. No. 371,902 '21 Claims. (Cl. '35--'19) The present invention relates to new and improved apparatus for detecting and demonstrating heat efiects and to methods for using the same.

Traditionally, devices such as thermometers, thermocouples, radiometer bulbs, concave mirrors, and the like, have been used to demonstrate heat phenomena. Apparatus of these prior categories have suffered from one or more disadvantages such as cost, inconvenience and fragility. For certain demonstrations, such as the focusing of infrared heat rays, relatively strong heat sources have been required. For demonstrating the many heat effects of interest moreover, an expensive assembly of apparatus has been required. For these reasons, many important or interesting heat experiments could not easily be carried out by the individual student or experimenter, and were confined to lecture demonstration. Also, on the lecture platform, many such demonstrations involving a thermometer or meter reading, have been poorly discernible at a distance, or have lacked visual appeal.

it is an object of the instant invention to fill the need for inexpensive, compact and dependable equipment capable of visually illustrating heat eflects in a striking manner, and otherwise to overcome the disadvantage of prior methods mentioned above. A further object is to teach the production and use of various elements and constructions for demonstrating what broadly may be termed heat effects. Still further objects of the inven tion, as well as its advantages, will be apparent from the balance of this specification, as well as the appended claims and the accompanying drawings, in which:

Figs. 1 to 5 inclusive show the structures of detectors of the invention in cross section; a

Fig. 6 shows, in cross section, a configuration of detector elements useful for the demonstration of frictional heat;

Figs. 7, 8 and 9 show in cross section various structures of the invention adapted to demonstrate the variable thermal conductivity of metals;

Fig. 10 illustrates a page or card incorporating one of the structures of Figs. 7, 8, or 9;

Fig. 11 illustrates application of the invention to forming of shadows in infrared radiation, as adapted to book form;

Fig. 12 shows in plan view the appearance of a flat sheet or card which may be folded to produce the structure shown in Fig. 11;

Fig. 13 shows in cross section a structure of the invention adapted to show the relative infrared reflectivity or absorptivity of various surfaces or colors or their reradiation characteristics;

Fig. 14 illustrates incorporation of the structure of Fig. 13 into a foldable arrangement, as for use in a book;

Fig. 15 shows a foldable reflector of the invention used to focus infrared radiation;

Fig. 16 shows in plan view the stamped sheet or card which may be folded to produce the reflector support illustrated in Fig. 15;

Figs. 17 and 17a illustrate alternative infrared focusing apparatus foldable into the pages of a book;

Fig. 18 shows in plan View the stamped sheet or card structure from which the apparatus of Fig. 17 may be formed by proper folding;

Fig. 19 illustrates an apparatus similar to that of Fig. 17, but adapted to the focusing of infrared radiation in sunlight;

Fig. 20 illustrates a foldable apparatus for demonstrating the relative infrared transmittancies of various materials;

Figs. 21, 22 and 23 illustrate three structures of the invention, in cross section, adapted to demonstrate a variety of electrical heating effects, the forms shown being alternative embodiments;

Fig. 24 shows in plan view one surface of a sheet-like structure of the invention adapted to demonstrate heating effects in series and parallel circuits;

Figs. 25, 26, 26a, 27 and 27a illustrate experiments in convective heat transfer;

Fig. 28 illustrates incross section a structure of the invention adapted to demonstrate heating effects produced by chemical reaction;

Fig. 28a illustrates a structure adapted to demonstrate heat effects produced by chemical reaction;

Fig. 29 illustrates use of the invention in demonstrating heat effects produced by internal friction;

Fig. 30 is a spectral reflectance curve of the double iodide of mercury and silver in the near ultraviolet and visible wavelength range;

Fig. 31 is a vertical sectional view of the new heat source of the invention taken at line 31-31 of Fig. 31a; and

Fig. 31a is a view of the base of the new heat source shown in Fig. 31.

Briefly, the above-mentioned and related aims are achieved by employing specially adapted materials containing a chemical substance or substances capable of changing colors upon suitable change of temperatures, thereby to illustrate the effects of heat or change of heat. The heat, or heat change, may result from thermal radiation, may be conducted by a heat-conductive sub stance, generated by an electromagnetic field, carried by a convection current of a fluid such as a gas, result from a chemical reaction, or result from the flow of an electric current through a resistance. Sheets of these es-. pecially adapted "chromo-thermosensitive materials, as well as auxiliary apparatus and constructions, as more fully described below, can easily and conveniently be assembled in a compact kit form or can be bound into a book or book-like structure which, when opened, is immediately ready for demonstration purposes.

Chemicals such as the double iodide of mercury and silver (Hgl .2AgI), or the double iodide of copper and mercury (HgI .2CuI), or mercuric iodide-silver chloride (HgI .2AgCl), which change colors at certain given temperatures, as used with the present invention, are well known at the present time. The preferred substance for use with the invention is the double iodide of mercury and silver mentioned above because it undergoes color change at the relatively low temperature of about 50 C., and is therefore especially sensitive, and because of the contrast and attractiveness of its color change, from bright yellow to cherry-red, as the temperature is raised. Heat-sensitive compounds may depart to a considerable extent from this stoichiometric composition in dicated above, yet remain useful for the purpose of this invent-ion. The broad scope of this disclosure includes the use of compounds which will not revert to their original color upon cooling although, obviously, such ingredients are not preferred since they can only be used once.

The heat-sensitive compounds of silver and mercury, including the preferred double iodide, may be observed to darken, i.e., to become overcast with a gray tint upon prolonged exposure to bright light, particularly light strong in actinic rays, such as direct sunlight. Fortunately, as I have observed, it is one of the additional remarkable properties of these materials that the gray overcast, if not extreme, will disappear completely when the material is stored in the dark for a period, such as overnight. This process may be accelerated by heating.

The gray material is probably silver, produced by decomposition of a minute amount of the silver halide, and the mechanism of restoration is probably the displacement of mercury from mercuric iodide by the silver, followed by evaporation of the mercury, according to the equation, in the case of the double iodide,

Incorporation of the sensitive pigment in any of a variety of vehicles does not of itself appear to alter this sensitivity to light. In ordinary experimental applications the'effect'is quite negligible, but in certain cases where exposure to bright light such as direct sunlight is sometimes unavoidable, as in signs, displays and signalling panels, it is desirable to avoid appreciable darkening, even if temporary.

As a feature of this invention, filter means are provided by means of which such deterioration may be substantially completely prevented. The particular filter means herein exemplified have reference especially tothe double iodide of silver andmercury. As indicated by the spectral reflectance curve of the double iodide in the near ultraviolet and visible wavelength range, shown in Fig. 30, this material is highly reflective in the region from about 560 millimicrons to at least the long wavelength end of the visible range. Toward shorter wavelengths, reflectance drops off sharply to an effective cutoff at about 500 millimicrons, thereafter being highly absorptive, and leaving only a small residue of reflection probably attributable to specular reflection from the crystal faces. The data of Fig. 30 was obtained on a Beckman DU spectrophotometer, using MgCO as a 100% reference standard, the compound being at a temperature of about 20 C.

Photochemical deterioration of the double iodide, being the result of absorbed photon energy, is attributable to wavelengths shorter than about 560 millimicrons, and especially to wavelengths shorter than about 500 millimicrons, down to the short wavelength limit of the sunlight or other incident light. Accordingly, the invention provides for the use of filter means overlying the crystals of the sensitive compound, having preferably a transmission characteristic substantially matching the reflection characteristic of the compound. Such a filter will selectively absorb the damaging wavelengths before they reach the sensitive material, yet will leave substantially unaltered the color presented by the material to the eye. For the double iodide of mercury and silver such a matching filter will be yellow, of course, like the compound itself below transition temperature. Since the change of color of the double iodide above transition temperature is substantially the result only of a shift in cutoff toward longer wavelengths, the filter leaves unchanged also the color above transition. Although a substantial match in spectral characteristics of the compound and filter is the ideal condition, practical benefits are derived even when the deviation from perfect matching is considerable. In fact, using practical filter materials, I have been able toexpose detector surfaces for long periods of time in full sunlight without visible deterioration. In any case, how ever, the filter medium itself must be such as not to suffer appreciably from bleaching or deterioration in bright light. As an example, a filter material having a transmission characteristic equal to the well known K2 photographic filter has been found suitable. Other suitable filter ma;

terials may be made with readily available commercial d es.

The filter may take the form of a tinted layer of lacquer, or the like, deposited over the thermosensitive layer and having suitable transmittance characteristics; or it may take the form of a tinted plastic layer super-imposed upon or laminated onto the sensitive surface, incidentally to provide physical protection of the structure. An alternative method involves incorporation of a dye of suitable transmission characteristics in the vehicle in which the sensitive pigment itself is distributed. As another alternative, in cases where the detector surface is substantially entirely illuminated by special local lighting means, the filter means may be applied next to the light source itself.

At least partial protection may alternatively be provided by a filter means absorptive of ultraviolet rays, but substantially transparent to the visible. Such a filter would in itself appear colorless.

As an alternative or supplementary-method of the invention, for preventing photochemical discoloration of the mercuric silver halide, use is made of the discovery that spontaneous color-recovery, after darkening in light, is greatly hastened by the presence of minute amounts of moisture. I have found that if the air around the detector, or the detector itself is kept slightly humidified, no color darkening may occur at all even during extreme exposures. Accordingly, any of the layers of the sensitive constructions shown in Figs. 1 to 5 may embody small amounts of humidifying agents, such as glycerol or other moistening agents known, for example, in the food and tobacco industries.

In the past, thermosensitive pigments have sometimes been applied to relatively massive bodies, such as bearings or engine bodies, to observe heat distribution. Also, such substances have been painted on relatively thick sheet material, thereafter to be attached to heated bodies. Such constructions are relatively very insensitive to minute heat changes because the thermosensitive layers are intimately coupled to relatively large thermal capacities, and because lateral heat conduction in the support would spread out and dissipate minute amounts of heat which might be applied to small local areas of the thermosensitive material. I According to one construction of the invention, the scnsitivity, or response to minute heat changes, is greatly enhanced first, by making the thermosensitive layer very thin, preferably not to exceed about .001, whereby low thermal capacity is obtained and lateral heat conduction minimized, and second, by placing it upon a layer of insulating material, whereby physical support is provided without materially increasing lateral conduction. A further effect of the insulator is substantially to eliminate radiative and convective heat loss from the rear surface of the thermosensitive layer.

Preferably, the insulating material is highly porous and of the lowest possible average density. This type of construction is illustrated in Fig, l of the drawings as comprising a base of insulation 2 such as cellular polystyrene, foamed polyester di-isocyanate compositions, pith, cork, or the like, surmounted by a layer 1 of the heat-sensitive substance employed. This substance may be attached to the base 2 by being dispersed in a film of a suitable organic lacquer such as, for example, a cellulose nitrate or acetate base lacquer or a composition formed by dissolving a vinyl acetate in acetone. Other materials such as common sodium silicate (water glass) or various silicones 'can be used as binders in this manner. Alternatively, satisfactory coatings of heat-sensitive materials can be created by dusting the dry thermosensitive substance upon tacky layers of any of the above binders deposited on insulating bases. The shape of the sensitive area created may conform to any desired configuration.

A second method of obtaining a heat-sensitive material responding to small heat changes comprises embodying a sensitive substance, as indicated above, within a layer 3 of a film forming material, such as shown in Fig. 2,

material.

prior to the formation of a film from this material. Preferably, the final film employed with this construction possesses a low-density, heat insulating structure and is composed of a substance such as cellular polystyrene, foamed di-isocyanate polyester resin, 'or the like. Alternatively, the final film is of a material which is not foamed, such as cellulose acetate, vinyl acetate, or the like, and in this case the film is made thin, not to exceed .005", and preferably .001" or less, so as to minimize heat capacity and lateral spread of heat.

A third construction for obtaining highly effective temperature-sensitive sheets in accordance with the invention consists of placing a heat-sensitive substance in contact with a fibrous fabric of felted, matted or woven nature. Such fibrous materials are of light mass and have low lateral heat conductance. The sensitive substance may be held. in a fixed position merely by adhering within the interstices of the fibers involved, or the fabric may previously have been rendered slightly tacky by suitable pretreatment. A construction of the broad category herein described is illustrated in Fig. 4 of the drawing as consisting of a fabric base 5 having an adherent surface layer 6 of a heat-sensitive compound. If desired, this heat-sensitive compound can be embodied in a thin film Go, for example of plastic, as shown in Fig. 5, the sensitive film being then similar to film ll of Fig. 1. The sensitive film is then caused to adhere against the fabric 5 as indicated in the figure, by means of an adhesive layer, not shown.

Any of the constructions shown in Figs. 1, 2, 4 or 5, or described in discussingthe embodiments illustrated in these figures, can be used in conjunction with a reflective layer to aid in preventing radiative loss of heat from the sensitive constructions indicated. Such a reflective layer 4 is shown in Fig. 3 in conjunction with a sensitive layer 3 of the variety pictured in Fig. 2 of the drawings. Preferably, this layer 4 consists of a very thin deposit of aluminum or the like applied by known means such as vacuum evaporation. In some cases it is desirable or necessary first to coat the sensitive layer 3 with a protective lacquer or the like, before depositing the aluminum to prevent chemical reaction of the latter with the sensitive substance. With embodiments illustrated in Fig. l,the reflective layer can sometimes effectively be placed between the insulation 2 and the sensitive layer 1.

For various specialized illustrations of heat effects, it is frequently necessary to form further modified heatsensitive structures which are largely adaptations of constructions of general utility indicated above. Perhaps the simplest of these adapted structures is illustrated in 6 as consisting of a base insulator layer 8 surmounted by a heat-sensitive layer 7 which is in turn covered by a protective layer 9 of paper, cellulose acetate or similar Constructions of this variety are well adapted to illustrate, among other effects, how heat is generated by friction. When a common eraser, or the like, is rubbed over the protective paper layer 9, heat is generated causing the temperature-sensitive compound in the layer '7 to change color. if the layer 9 is paper, this change is visible when the paper is removed from the sensitive layer 7. If the protective layer 9 is transparent, it is not necessary to remove it to observe the color change; in fact, in this case, layer 9 is preferably permanently laminated on layer 7.

To demonstrate heat conduction, a heat-conductive rod 14-, or the like, can be attached by glue or similar means to a heat-sensitive sheet such as is shown in Fig. 7 as consisting of an inert base member 13 of any of the classes indicated above, including paper, coated with a heat-sensitive layer 12 in order to demonstrate the heat conductance of this rod. Preferably, layer 13 is of a thickness and material to minimize lateral spread of heat, but not so highly insulating as to prevent sufficient heat transfer from a rod 14 to sensitive layer 12. If desired,

a heat conductor 15 of this same type can, be placed within the surface of a material which'is covered With a heat-sensitive layer 17, as illustrated in Fig. 8. When this is done, it is preferred that the material 16 be a good insulator of the foamed type discussed above. It is also preferred to form the rod 15 in a rectangular cross section so that its top surface is flush with the surface of the material 16 under the sensitive layer 17. This is shown in Fig. 9 of the drawings. At times it is preferable merely to attach a heat conductor on top of or depressed within the sensitive material as herein described so that in use the conductor can be actually seen. In this case a progressive color change in the sensitive ma terial is observed along the border of the conductor.

Various devices can be employed utilizing the above constructions to illustrate the different heat conductivities of different metals. One such device is shown in Fig. 10 as consisting of a supporting construction 22 within or upon which are disposed rods 18, 19, 20 and 21 of metals having different heat conductivity, as indicated above. These rods terminatein a common block 23 of a material having good heat conductivity, such as a copper or brass, disposed within a recess in the support 22, as shown in the figure. In use, heat from a suitable source, which may be a common match, cigarette lighter, or the like, but which is preferably the heat source of Fig. 31 to be described hereinafter, is applied to the block 23 and is conducted by the rods 18, 19, 20 and 21. As heat conduction proceeds, various areas 24 of the supporting construction 22 progressively change color at varying rates showing the diiferent heat conductive properties of the different metals.

For demonstrating the heating effects of various electrical circuits, adaptations of the invention as shown in Figs. 21, 22, 23 and 24 canbe employed. Perhaps the simplest construction for this purpose is that shown in Fig. 21 of the drawings, consisting of a support member 99 provided upon one surface with a heat-sensitive layer 100 and upon the other surfaces with an electrically conductive layer 101. The closely related modification of Fig. 22 providing better thermal contact between the conductor and sensitive layer, uses a conductor 101 placed upon a support 99 so as to be separated from the heat-sensitive layer 100 by an inert layer 102 of lacquer, or the like, which can serve to hide the conductor 101 and/or in some cases to prevent the possibility of chemical interaction between it and the sensitive layer 100. .When the hiding function and chemical protection are not essential, layer 102 may be omitted, and sensitive layer 100 deposited directly over electrical conductor 101. It is also possible to dispose an electrical conductor 101 directly on a sensitive layer 103 of the type discussed in conjunction with Fig. 2 of the drawings, as is shown in Fig. 23. It is to be understood that these electrical conductors can be disposed within surfaces in the same manner in which the heat conductors shown in Figs. 8 and 9 are disposed within supporting bodies. Any of the broad types of heat-sensitive constructions shown and/or discussed in conjunction with Figs. 1 to 5 of the drawings can be used with electrical conductors as herein described. Although metal wires, sheets of metal foil, or metallized layers selectivelydeposite'd by plating or shaped by etching can be used as such conductors, it is preferred to employ layers of conductive material such as are commonly used in printed circuit work; These materials and their modes of application are described in the Bureau of Standards Circular 137 entitled, Printed Circuit Techniques.

The actual circuits used with the various electrical conductor constructions indicated in the preceding paragraph are, of course, capable of wide modification. For example, series of parallel circuits, or combinations thereof, maybe used to demonstrate basic laws of heat generation Within electrical'resistive'elements produced by the flow of current. In Fig. 24 of the drawings, two simple circuits are illustrated. Circuit 104 is a series circuit in which current passes from a terminal 117 to a broadened terminal contact area 116', thence through a lead 115' to various conductive sections 111, 112, 113 and 114, thence to the second lead 115' to a second terminal area 116 and a terminal 117 attached thereto.

The circuit shown in Fig. 24, and designated by the numeral 105, is of parallel configuration and employs terminals 117, terminal areas 116, and connecting leads 115 of the type used with the circuit 104. Itutilizes a plurality of parallel members 107, 108, 109 and 110 connected across the leads 115. Each of these members 107, 108, 109 and 110 is of different cross-sectional area.

The resistances of the various parts of the series and parallel circuits are different as a consequence of differences in cross-sectional area of the conductive sections 107, 108, 109, 110, 114, 113, 112 and 111. Assuming as the preferred situation, that the resistive composition, the thickness and the length of the sections in each of the circuits are the same, then the heat generated by each of these sections will vary depending upon the width of the conductor, and the relative heating effects within the sections of the two circuits shown will be observed to follow different laws.

Although the method shown in the drawings for varying resistance within an electrical conductor is to change its cross-sectional area, other means may be employed, such as, for example, the use of compositions of various specific resistivity. For example, conductors of different compositions can be attached to one another in series so as to indicate differences in heating effect.

When the various circuit means indicated are disposed beneath a heat-sensitive layer it is frequently advantageous to print upon the top of this layer various patterns conforming to the pattern of the conductor used. Constructions employing a heat-sensitive compound disposed over a single conductive layer can be used to replace glow lamps or for illustrative and advertising purposes. In another type of construction a plurality of conductive circuit layers are placed one upon the other and insulated from each other by appropriate lacquers, the composite layers being surmounted by a thermosensitive layer. Such constructions can advantageously be employed at times to form advertising or signalling constructions in which different latters or different figures are selectively or alternatively made to appear by selective circuit switching.

In another arrangement, a conductive resistance grid is printed on one face of the support card, and the other face is printed in a non-sensitive colored pigment matching the cold color of the thermosensitive substance. Upon this matching ground is printed a pattern or message in thermosensitive material. The conductive grid, when heated electrically, warms the support substantially uniformly and brings forth the previously invisible pattern.

It will be understood, however, that the prime application of this part of the present invention lies in the formation of kits and/ or books useful for the demonstration or display of heat effects, specifically the heat generated by different types of electric circuits.

Heating caused by electromagnetic induction can be demonstrated using those sheets indicated above as being provided with electrically conductive patterns by including the conductors employed within single or multiturn pickup loop circuits. Such constructions can be used in the detection of alternating fields.

The construction shown in Fig. 13 of the drawings has been developed to indicate visually the effect of reflection, heat-absorption and re-radiaticn by various bodies having different colors or surface characteristics. This construction consists essentially of a base member 33 having on one side a thermosensitive coat 34 and on the other side a plurality of coatings, exemplified by 35, 38 and 40, having dilferent infrared reflection and absorption characteristics. The thickness and the material of the base layer 33 areselected to minimize lateral heat spread, but the layer is not made so highly insulating as to prevent adequate heat transfer between any of the coatings 35, 38, etc., and the thermosensitive layer. Coating 35 may be aluminum foil, 38 may be a dull black paint and 40 may be any of a variety of surface colors or materials of interest relative to heat absorption. A portion of the metallic coating 35 may in turn be covered as shown with an area of dull black paint 36, and the area of black coating 38 may in turn be covered with aluminum foil 39. It is to be noted that any of the specific sensitive constructions shown in Figs. 1, 2, 4, or 5 of the drawings can be employed for the broad purpose for which the specific construction shown in Fig. 13 is used. Although this and similar structures can be mounted in any desired manner for exposure to heat radiation, it is preferred to mount them broadly in the manner indicated in Fig. 14 of the drawings. Here this construction is shown as positioned upon an upright element 44 attached at its base to a sheet 42 and attached at its rear portion by arms 45 and 46 to a second sheet 43 positioned at right angles to the first sheet 42. By virtue of this construction, the element 44 can be folded into a flat position as the two sheets 43 and 42 are either brought together or laid in the same plane. This broad type of parallelogram" construction is old in the art and has been provided for years with various toy constructions such as are specifically shown in the Gibson Patent 565,450.

In use, the element 44 of Fig. 14, embodying the structure of Fig. 13, is irradiated on the side carrying coatings 35, 38 etc., with any suitable infrared source, casting a shadow of the upright element 44 on the sheet 43 and applying heat to the coatings. Preferably, the source is that illustrated in Fig. 31, to be described in detail later. The response of appropriate portions of thermosensitive layer 34 is indicative of relative heat absorption or reflection by the various irradiated layers. Where coating 36 locally covers the aluminum coat 35, heat is seen to be readily absorbed and transmitted to the sensitive coat, in spite of the presence of the aluminum layer. Where aluminum coat 39 covers black coat 38, the latter is efiectively shielded from the heating effect in the area back of the former.

If element 44 is irradiated, alternatively, on the thermosensitive side, then color response in the appropriate areas thereof is dependent on extent of re-radiation from the areas 35, 39, etc. The greater the re-radiation (the black coat, for example, re-radiating much more strongly than the aluminum), the more theheat required to produce color change in the sensitive coat.

Use of the parallelogram type of construction is further illustrated in Fig. 12 as consisting of flat sheet sections 25 and 26 and a center panel section 27. In the modification of Fig. 12, a small area around 28 is cut from the panel section 27 so as to define the letter A for purposes of illustrating how infrared radiation casts a shadow upon appropriate sensitive materials, as earlier indicated. In this figure, the portion of the sheet 26 covering the area immediately beneath the opening around the letter at 28 is surmounted by a sheet 32 of heat-sensitive material, as previously indicated. The use of the sheet shown in Fig. 12 is quite clearly demonstrated in Fig. 11 of the drawings. Here it is seen that this sheet is positioned so that the portions 25 and 26 are at right angles to one another and that the section 27 is parallel to the sheet 26, being held therefrom by the small wall portion illustrated. The letter A defined by the cut-out area around 28 is herein shown as casting a shadow upon the heat-sensitive element 32 in the same shape as the cut-out area with the shadow 9 portion 30 corresponding to this A remaining unchanged in color. The source of radiation used for the purpose of casting a shadow can be any convenient source of infrared radiation. It is preferred, however, to use the small heat lamp source 29 which will be more fully explained later. In Fig. 11 it is shown that the sensitive element 32 can be mounted upon a page 26a immediately back of the sheet 26. In use in demonstrating heat effects, radiation from the lamp 29, directed through the open portions around 28 which define the letter A will cause the color-sensitive material disposed upon the sheet 32 to change color selectively, producing a shadow immediately back of the formed figure. This strikingly demonstrates the fact that invisible infrared radiation, like visible light rays, travels in straight lines.

Fig. 28 illustrates how the heat-sensitive sheets of the invention may be adapted to demonstrate the generation of heat in a chemical reaction. The construction, shown in cross section, is that of a card or sheet formed of several layers. The supporting layer 126 is preferably of paper card material, this being provided on one face with a thermosensitive layer 127. Permanently attached to the other side by a layer of adhesive, not shown, is a layer of noncombustible material 12d, preferably in the form of glass cloth. Layer 129 is a sheet of relatively thin paper provided on one side with a layer of pressuresensitive adhesive, not shown, by means of which it is caused to adhere to the noncombustible layer. The use of the pressure-sensitive adhesive permits rapid and convenient removal and replacement of the paper layer 129. The layer 129 is printed or otherwise selectively coated with an image or pattern by application of a solution or ink, which may be colorless, and which contains an oxidizing agent such as potassium nitrate. Layer 129 is prepared in this way preferably prior to adhesion to the noncombustible layer 128, usually being prepared by the manufacturer and supplied as a pad or a set of sheets.

When the paper layer 129 is ignited at any suitable portion of the inked image, which may be marked for this purpose, the paper burns slowly and progressively along the image area. The progress of burning may be observed by the heat image produced in the thermosensitive layer 127. When the image area has been completely burned, the paper layer 129 may be stripped from the noncombustible layer 128 and replaced with a fresh sheet. A typical inked pattern is that shown in Fig. 28a, the ignition point being designated by numeral 130.

The sensitivity and low lateral thermal diffusion of the sensitive detector constructions described adapt them ideally to novel experiments in the focusing of infrared radiation. Even when used with conventional focusing means, such as lenses, or mirrors 'which are figures of revolution, whether spherical, ellipsoidal or paraboloidal, they permit use of smaller and more compact optics, and the use of safer sources of much lower wattage than heretofore used. An electrical source of or watts, operated at a barely visible dull heat, is quite adequate for use with the apparatus to be described. Although sources of much lower wattage still may be employed to make all the desired demonstrations by reducing the emitting area of the source, the reduced size of any effective image obtained, being less striking to the observer, offsets advantages of the smaller source.

The invent-ion provides means, however, whereby even the optical focusing elements may be greatly simplified and reduced in cost, and whereby the optics, positioning means and detector may be made compactly foldable, as into the pages of a book. As a feature of the invention, use is made of focusing reflector elements which are figures of translation, rather than the usual figures of revolution, and which may be formed by the flexing of flat sheets of reflective material. As a preferred ar- 'ran'g'ement, the curved figure to which the reflective sheet is flexed is determined by a foldable shaping means '10 which includes a shaping edge cut to a desired mathematical curve.

Assuming the axis of the figure of translation to be positioned vertically, then focusing occurs only for the horizontal component of rays emitted from the source. The vertical component is attenuated, by divergence, in inverse proportion to path length between source and detector. Accordingly, the image is astigmatic, i.e., elongated in the vertical direction, and may suffer in intensity as compared with that given by the related figure of revolution of equal relative aperture, used with equal path length. Both of these disadvantages may be substantially overcome by selecting a source the emitting portion of which is itself elongated in shape, its length being placed parallel to the direction of translation of the reflector, and making the total path length to the detector relatively short. The vertical dimension of the mirror, moreover, relative to that of the detector, and of the source, should be adequate to prevent vignetting of any of the rays which could otherwise be traced from the source to the detector. Since the sensitivity of the detector permits the use of a small source, there is no inconvenience or danger in designing for moderately close proximity of the elements to each other, so as to reduce path length. Small scaling is in any event desired when incorporating the apparatus into a book.

Figs. 15 and 16 illustrate the construction of a foldable focusing device which incorporates the aforementioned principles. As seen in Fig. 15, the general form of the apparatus is a hollow rectangular parallelepiped with open ends, comprising the faces 47, 48, 49 and 50. The entire structure, less the flexed mirror sheet 48b itself, may be stamped from a flat sheet, having the appearance prior to folding shown in Fig. 16, wherein similar parts are designated by similar numerals. Folding lines are indicated in Fig. 16 by dotted lines and the cutout area is designated as 48a. An end tab 51, adjoining face 47, is cemented to the edge of face 50. The mirror sheet 48b, providing a surface of high specular reflectance in the infrared, may comprise a vacuum-aluminized acetate sheet or a' relatively heavy gage of aluminum foil secured to a manila card backing, or the like. The mirror is fitted into a recess in the face 48, and is accurately shaped to the desired curve by stamped curved edges 52 and '53 provided respectively in faces 47 and 49. One edge of the mirror sheet fits into notches 54 and 55; the opposite vertical edge of the mirror sheet is acted upon by a pressure tab 56 in such a manner as to retain the mirror firmly and accurately against the shaping edges. Pressure tab 56 derives its action merely by the tensional force along its folding edge. Recesses 57 and 58, provided in the faces 47 and 49, respectively, allow free rotational movement of the pressure tab. In order to elevate the parallelepiped sufliciently to raise the mirror sheet above the supporting table, the structure is pro vided with three feet 59, 60 and 61, as shown. When the mirror sheet is removed from its recess, the parallelepiped may be collapsed into a thin flat structure.

The curved edges 52 and 53 may be circular, a source 61a and detector being positioned relatively close to each other on either side of the axial line A-A of the circular cylinder and preferably centered at about half the height of the mirror. Alternatively, the curved edges may be ellipses, the source and detector each being placed at one of the foci. Means may further be employed whereby the structure of Fig. 15 may be properly positioned with respect both to the source and the detector. Inasmuch as the rays to be focused are invisible, and inasmuch as the geometry of the system must be fairly accurately defined to achieve the necessary concentrated image, the provision of positioning means is rather essential. This may comprise simply a printed sheet 6112 of paper which is laid upon the table top, and which is marked with the positions at which the source, detector and focusing mirror are to be located. These elements may accordingly be assembled in proper place directly on the locating sheet. Theunderlying sheet may also be marked with a diagram showing the path of the rays.

' Fig. 17 illustrates a structure more specifically adapted for incoporation into the pages of a book. Panels 62, 63, 64, 65, 66 land 67 may be formed by stamping out a single piece of flat sheet material, having the appearance, prior to folding, shown in plan view in Fig. 18. Panels 63and 64 provide a recess 68a with a curved mirror-shaping edge 68, into which a mirror sheet 69 is accurately fitted and wherein it is secured at one end by a pressure tab 70. The opposite vertical edge of the mir ror sheet is preferably permanently secured to panel 64 by stapling at 71, orby any other suitable means. Vertical panel 66 and horizontal panel 65 are apertured to provide a positioning space 72 for locating a radiant heat source 73. The panel 66 provides, in addition, support for a vertical detector tab 74, secured at 75 by stapling, as shown, or secured by any other suitable means. The detector tab 74 carries a thermosensitive detector area 76, preferably on the tab face directed toward the reflector 69. Horizontal panel 65 carries a printed curved line 77 representing the mirror, the curved line actually marking the projection of the mirror figure upon this panel. Also printed upon this panel is the path of selected rays 78 from the source to the detector.

The embodiment of Fig. 17 illustrates the positioning of source and detector relatively far apart, as compared with the width of the mirror. Accordingly, the shaping curve 68 is preferably elliptical. In an alternative arrangement, source and detector may be positioned relatively more closely together and a circular shaping curve used.

A vertical supporting means 79, shown here as secured at the base to a page or rear book cover underlying panel 67, fits securely into an aperture in panel 62, thereby maintaining the latter erect and assuring proper orientation of the various planes of the structure.

When it is desired to fold into closed position the panels or pages 62 and 67, the support 79 is dislodged and folded out of the way, the free end of the mirror sheet is dislodged from its recess, and the radiation source removed. The entire structure is then collapsible into very thin form.

Using the circular or elliptical reflector of the invention and an elongated radiation emitter oriented parallel to the direction of translation, the image produced will be a stripe parallel to this same direction also. Assuming the source and detector to be spaced at equal distances from the reflector, the width of the image will be equal to the width of the emitter.

An alternative, although less versatile and less accurate arrangement for producing a flexed mirror operates without use of curve-shaping edges, the curve merely being determined by the elastic properties of the mirror sheet structure itself. An arrangement illustrating the principle is shown in Fig. 17a, depicting again a foldable arrangement which may be incorporated into a book. Similar numerals are used to designate parts having similar functions to those shown in Fig. 17.

Panels 62 and 67 may be the adjoining pages of a book. Panel 62 may be wholly of reflective material, e.g., a thin sheet of brightly polished metal or a sheet of vacuum-metallized plastic. Alternatively, panel 62 is a supporting structure, e.g., a paper card covered, at least in part, on its inwardly facing surface, with a reflective area 69 of suitable material, such as aluminum foil or metallized plastic. A tensioning band B or other suitable tensioningmeans is secured at one end in or near the book binding, as shown, and at the other end is provided with means detachably securing it to the outer, marginal portion of the panel ,62 so as to hold the panel in suitably flexed condition, as shown.

The inwardly facing surface of panel 67 is provided at least in part with a thermosensitive detector area 76.

. H. 12 By suitably flexing the reflective sheet, also by suitably adjusting the position of the detector area 76 with respect to the reflector, and properly positioning a radiant source 73 with respect to both of these elements, a focused image of the radiation-emitting element may be obtained on the detector area. I have found that suitable flexing of the mirror will produce a curve sufliciently approximating the necessary ellipse to give quite satisfactory results.

As an aid in positioning the source, mirror and detector panel properly with respect to each other, these elements may be set down upon a flat locating sheet 65 suitably marked to show correct placement. The sheet 65 may also be marked to show the paths 78 of selected rays. if desired, this locating sheet may likewise be made integral with the book-for example, it may be formed of the same sheet of paper of which panel 67 is made, comprising a downwardly folding lower extension thereof.

Upon removing the source 73 and upon detaching band h from panel 62 and folding sheet 65 upward, the entire structure may be folded flat and compact.

The focusing devices previously described are readily adaptable to focusing of infrared radiation from the sun. The suns rays being substantially parallel, the mirror is shaped in this case to a parabolic curve. The essential optical geometry is shown in Fig. 19, wherein the reflector 80, a parabolic figure of translation, is oriented to receive parallel rays from the sun, exemplified by 81 and 82, in such direction as to direct them upon the focal line PP. The detector tab 83 is positioned to have the focal line lie substantially in the plane of the thermosensitive area, whereby a sharp line image 84 is produced, of height equal to the vertical mirror dimension or thevertical detector dimension, whichever is the smaller. The thermosensitive area may be placed on the tab surface facing the mirror or, if the tab is a moderately thin card, on the opposite face. The latter arrangement has the advantage that glare at the focus does not interfere with easy observation of the color change. In the latter case also, the detector tab surface facing the mirror may be coated with a dull black to increase sensitivity.

By way of example, the means used for shaping the mirror, for positioning it relative to the detector, for showing the ray paths and for providing for incorporation into a book may be quite similar to those shown in Fig. 17. A parabolic curve is substituted for the circular or elliptical curve of Fig. 17; the heater source shown therein is removed, and the detector tab 74 is shifted on panel .66 to center it in front of the mirror. The detector tab being only a fraction of the Width of the mirror, it does not excessively block the suns radiation.

Suitable means must be provided for orienting or aiming the entire structure with respect to the direction of the suns rays. Unless this is done, focusing may be awkward or difficult to achieve. Such means may take the form of a small aperture in one of the vertical panels of the structure of Fig. 17, or in a specially provided vertical panel in the structure, together with a suitably placed mark or aiming spot on another vertical surface removed by some lateral distance from the aperture. The aperture and the mark are disposed to lie on a line parallel to the focusing direction of the parabolic surface. Accordingly, upon orienting the apparatus so as to direct a beam of sunlight through the hole and upon the mark, the apparatus is found to be properly aligned for focusmg.

Referring again to Fig. 19, a simple sighting arrangement is shown, consisting of a small aperture 85 in the detector tab, and a spot 86, for example of white paint, marked directly upon the reflector in a suitable position with respect to the aperture.

While the parabolic mirror shape idealya circular mirror may be found almost equally effective, the focal length being then substantially half'the radius of curvature. This is particularly true when detector sensitivity permits use of a low relative aperture, i.e., ratio of mirror width to focal length.

The relative transparency of materials to infrared radiation may be demonstrated by the arrangement illustrated in Fig. 20, which is adapted to be used in book or kit form. Panels 87, 88, 89 and 90 may be produced by properly stamping and folding a single sheet of material, using the techniques referred to earlier. A disc 91 of similar material is rotatably mounted on vertical panel 89 by means of an eyelet 92 or other suitable means. The disc 91 is provided with a number of similar apertures near its periphery, as exemplified by numeral 93, and each aperture is closed, as by cementing upon the rear surface, by a rectangular piece of material, indicated by the dotted lines, transparent to a greater or lesser extent to infrared radiation. The panel 89 is provided with two uncovered apertures, one of these being 94 and the other an aperture, not shown, lying directly behind the selected aperture designated as 93 and being at least as large as the latter in size. A thermosensitive area or detector means 95 is provided behind the panel 89, and may be mounted either by attachment directly to the rear surface of panel 87 or as a separate sheet, e.g., a portion of the adjoining page 87a.

Upon directing infrared radiation from a suitable radi ant source 96 equally upon the uncovered aperture 94 and the selected covered aperture 93, a relative measure is obtained of the infrared transparency of the material covering aperture 93, by noting the relative detector respouse in the areas 97 and 98. By rotating any desired aperture of the disc 91 into position alongside aperture 94, the relative transparency of any of the several materials may be determined. Materials suitable for mounting on disc 91 may include the following: glass; plastics of various kinds including Lucite, polystyrene, polyethylene, cellulose, acetate, etc.; hard rubber; mica; and mineral crystals such as quartz and salt.

In an alternative arrangement, the transmission wheel 91 of Fig. 20 is combined with a focusing apparatus, for example, that of Fig. 17. In this case detector tab 74 would be made somewhat larger so as to accommodate thereon the rotatable transmission wheel. Such a wheel would be adapted to present selectively, between the detector surface and the mirror, any of a variety of materials whose transparency to infrared rays it is desirable to test. For the ordinary focusing experiments, however, the wheel would present simply an open aperture before the detector.

In a further alternative arrangement, several apertures covered by materials of varying transmittancy are provided in a leaf of a book. A thermosensitive area is provided on an immediately adjoining leaf, facing the apertures. Irradiation of the combination, with the two leaves slightly separated, if desired, and the apertured leaf positioned between the heat source and the thermosensitive area, provides differing responses in the thermosensitive area indicative of varying infrared transparency.

The source of heat radiation illustrated in many of the preceding figures of the drawing is best shown in Figs. 31 and 31a as comprising a shell 134 of ceramic or similar material which is provided with a thickened base portion 135, an upper chimney-like aperture 136 and a slot-like radiation opening 137 in the shell. Within the base 135 there is positioned a terminal block 138 carrying a central heater rod 139 and terminals 140. Upon this rod 139 is wound a resistance wire 141, the ends of which are connected to the terminals 1401 The upper end of the heater rod is inserted into a conduction bar 142 which leads to a terminus 144 exterior of the shell 134 by way of an external aperture 143. Preferably, the terminal of this conduction bar 142 is recessed within a grooved portion of shell 134 so as to minimize danger of burns to the operator.

The heater of this inventionis designed to comprise a heat source of low wattage, preferably within the range of 10 to 15 watts. Its novel design adapts it to supply heat by radiation via the opening 137, by conduction via the conduction terminus 144, and by convection via the chimney 136. When used with the heat-conduction apparatus of Fig. 10, for example, its conduction terminus 144 is held in contact With the terminal block 23 of that apparatus. All of the heat-sensitive layer constructions described above will easily respond to a heater of this wattage range in the demonstrations described in this specification. Certain of the elements such as, for example, the electric circuits indicated in Fig. 24 can, of course, be sensitive to much smaller levels of thermal energy. With the specific circuits shown in this figure, about one watt of power for each circuit is normally satisfactory for demonstration purposes. Moreover, the voltage of a single dry cell, about 1.5 volts, is entirely adequate, enabling the entire device to be used safely and conveniently even by those unfamiliar with electrical circuits. In other experiments in the detection of small amounts of electrical and radiant energy the detectors of the invention have responded easily to energy of the order of .011 watt.

The heater or radiant source shown in Fig. 31 of the drawing can be employed in a number of heat experiments which do not require especially adapted heat-sensitive constructions but merely use a heat-sensitive sheet as illustrated or described in connection with Figs. 1 to 5, inclusive, of the drawings. As an example of this, in Fig. 25 a heat-sensitive sheet 118 is shown placed over the chimney of the heater 119 so that heated air ascending from this chimney flows over the surface of the sheet 118 forming an area 120 which changes in color due to increase in temperature, illustrating clearly that hot gas rises, and showing the pattern of the air stream. If desired, the heated air stream from such a chimney may be split and deflected by means of a small V-shaped cardboard sheet 121 so as to form a substantially U-shaped pattern 122, as is shown in Fig. 26 of the drawings. Or, alternatively, the heat from the top of the chimney of the lamp 119 may be carried along an inverted V-shaped trough 124 so as to rise outward along a curved path 125, as is shown in Fig. 27. An alternative construction for demonstrating the same experiment is shown in Fig. 27a and comprises a heat-sensitive sheet 118 formed with a slit 1241: turned back so as to form a ridge 124a, as shown in Fig. 27a. Ridge 124a and the adjoining portion of card 118 form an inverted trough, and provide a structure easily foldable within the pages of a book. Fig. 26a shows another book page constuction, for showing the same experiment as that Fig. 26. A thermosensitive sheet 118 is provided with an upwardly folding lower portion 121a. A pair of ears 123 fit in perforations in sheet 118 to maintain the portion 121a in a trough-like configuration.

The new heat-sensitive sheets of the invention can be used to show how heat is actually removed from a warm surface of higher than body temperature by the hand. To illustrate this, a heat-sensitive surface, as indicated above, is preheated to a substantially uniform degree so as to change color; then the hand is placed upon the sheet for a few moments. Upon removing the hand, the area of the sheet which was contacted is found to have reverted to its original color.

A similar experiment which can be performed, using the novel heat-sensitive sheets herein described, illustrates vividly the warm and cold feel of different materials. It is well known that if two objects, one a thermal insulator and the other a thermal conductor, are touched by hand, the insulator atroom temperature will feel relatively warm to the touch, whereas, the conductor at the same temperature will feel quite cold. This effect is a consequence of the fact that the conductor removes heat quite rapidly Whereas the insulator does not. To demonstrate this, in accordance with the invention, a heat-sensitive 15 sheet, in use, is brought uniformly to a temperature at which it changes color. .Two objects of .similar shape, one an insulator and the other a conductor, are placed in contact with the surface of the sheet for a few seconds; then both are removed. Beneath the conductor the sensitive material is found to have reverted to its original color Whereas beneath the insulator substantially no change will have occurred.

The cooling effect of evaporation may be illustrated as follows: A pattern of volatile liquid such as water is distributed or painted upon a sheet of thermosensitive material such as described before, on either the thermosensitive or the reverse side. Preferably, the surface is of such material as to be comparatively wettable by the liquid so as to retain the pattern. Then the wetted structure is heated by any suitable radiant heat means,

for example, an infrared bulb or the heat source shown in Fig. 31. The area covered by the liquid will retain its original color until such time as substantially all of the liquid has evaporated, while the remaining area not covered by the liquid will change in color ,quite rapidly.

The heat-sensitive sheets of the invention can be used to demonstrate the fact that heat may be generated by internal friction produced, for example, when a metal object is bent back and forth about the same spot. This is clearly illustrated in Fig. 29 of the drawings. Here a common paper clip 131 is spread out and held down firmly with one end against a thermosensitive area on a supporting surface 132. The other end of the clip is gripped by the hand and bent back and forth rapidly so as to develop a certain amount of heat by internal friction. This heat of friction will cause the area 133 of the card 132 immediately adjacent the portion being flexed to change color.

Those skilled in the art to which this invention pertains will realize that the herein described and disclosed invention is capable of wide modifications. As an example of such modification, the sensitive surfaces described may be increased in sensitivity by being used Within a vacuum chamber. Similarly, they may be preheated to a temperature immediately below the point at which the color changes for many experiments. This preheating may be accomplished by internal resistance elements built within the cards or by other means which will readily be apparent. Such modifications, insofar 'as they are within the scope defined in the appended claims, are to be considered as a part of the instant inventive concept.

I claim as my invention:

1. An experimental and demonstration device in the form of a book for experiments in heat phenomena ineluding: page binding means; a plurality of pages bound by said binding means including at least one page surface bound permanently as a part of said book and provided with an area of thermosensitive material responding reversibly to temperature change by change of color; and cooperating shadow-producing means collapsible within said book and secured thereto as a part thereof and adjoining said area of thermosensitive material, said cooperating shadow-producing means bounding zones differing in infrared transparency and thus differing in optical properties to alter the distribution of infrared rays derived from a remote radiant heat source and impinging upon said area after traversing the position of said cooperating means, whereby said area is differential- 1y heated and changed in color to produce an image determined by the optical properties of said cooperating means.

2. A device as in claim 1 in which said cooperating means is an apertured sheet of a material substantially opaque to infrared rays adapted to produce an image of the aperture pattern on said thermosensitive area by interposition of said sheet between said thermosensitive area and said radiant heat source. i i i I H .3. A demonstration and display device comprising: a relatively thin substantially planar supportlayer provid ing relatively extensive front and rear surfaces; layer means applied and fixed permanently to saidrear surface in selected portions thereof, said applied layer means cooperating with. an external'source of energy to inject heat into said support layer in areas covered by said layer means, the resulting heat being transmitted through said thin support layer to heat corresponding areas of said front surface; and a very thin thermosensitive layer fixed permanently to said front surface and comprising a thermosensitive substance changing color reversibly upon change in temperature induced by said heat trans mission to said front surface and thus signalling said in; jection and transmission of heat into and through said support layer.

4..A demonstration and display device as defined in claim 3 in which said layer means applied to said rear surface comprises a plurality of layers of materials differing from each other in their heat absorption properties applied to laterally spaced zones of said rear surface.

5. A demonstration and display device comprising: a.

relatively thin support layer providing relatively extensive front and rear surfaces; layer means applied locally and adhered fixed to said rear surface to define a pattern thereon, said layer means being adapted to cooperate with an external source of energy acting transiently to heat the material of the adhered layer means, the heated material locally injecting heat into said supportlayer for transmission as a heat pattern to said front surface; and a thermosensitive coating adhered fixedly to said front surface and comprising a thermosensitive substance changing color reversibly upon change in temperature in said heat pattern as a result of changes in energy received by said material from said source, said thermosensitive substance being present at least in zones of said front surface opposite the pattern of said applied layer means to signal said local injection of heat into said support layer and said transmission of said heat pattern. 6. A demonstration and display device as defined in claim 5 in which said relatively thin support layer is made of a material of low thermal conductivity to mini; mize lateral heat conduction beyond the edges of the pattern of said layer means, and in which said thermosensitive substance is present in zones beyond said heat pattern whereby the image produced by said heat pattern emerges transiently against a ground of the characteristic color of said thermosensitive layer when unheated. I

7. A heat-responsive image-forming device for demonstration and display purposes comprising: a relatively thin sheet having front and rear surfaces and made of a material of relatively low thermal conductivity to minimize lateral heat flow therein in the direction of said surfaces; a heatable layer means fixed to said rear surface as a pattern in contact with areas of said rear surface that are spaced from each other, said patterned layer means being heatable by an energy source'to inject a corresponding pattern of heat into said thin sheet, said heat pattern producing a corresponding front heat pattern at said front surface; and an extensive-area thermosensitive layer fixed permanently to said front surface throughout an area opposite and coextensive with said spaced areas of said rear surface and the spaces therebetween, said thermosensitive layer comprising a material reversibly changing color in response to temperature change to produce a heat-created colored image of said spaced areas against a background of the characteristic color of said thermosensitive layer when unheated, said heat-created image existing only so long as said heatable layer means is heated and having boundaries determined by and sub; stantially duplicating the boundaries of said spaced areas, said image being viewable from a position in front of said sheet. 8,. A device as defined in claim 7 in which said spaced properties adapted to alter the si at heat into east t u h aidsun said heat tr ansmission to said '17 areas of said rear surface are respegtively covered by ;two materials differing from each other iri heat absorption properties and thiisadapted to be differentially heated by said source. V

9; An experimental and demonstration device in the formof a book for experiments in heat phenomena including: page'binding means; a plurality of pages including at"-le'ast'or1 e page surface provided with area of 'thermosensitive material responding reversibly to temperature change by change of color; cooperating means, collapsible vvithin said book and securedthereto, adjoining said area of thermosensitive materialand haying optical distribution of infr ared rays derived from a remote radiant heat source andirnpinging upon said area, whereby said areaf'is differentiall y -lieatedand changed color to produce an image determined by the optical properties" of said cooperating means, said cooperating ir'iea'ns includingfa flexible area of material having high specular reflectancelin the infrared; and meansfof'fiexing and retaining" said flexible area in a curve adapted to receive rays from said radiant eat source and to focus said ra s'u dnsarithemije sensitive area. i.

19 A demonstration and display deyice comprising: a relatively thin substantially planar support layer providing relatively extensiye front and irear surfaces'i layer means comprisinga metallic 'layer applied to said rear surface in selcted'fjaorticbns thereof, ffsaid layer means co operating with ari external source of energy to ectheat into said support :layer in areas coyeredby said layer meagre, the resulting' heat being transmitted said thin support layer "t olheat corresponding areasjof said front surface; and a very thin ag ee ents-la e applied to said front surface 'and comprising a thermoseri sitive. useshaaainasslor reve s b yru ashaa in perature induced by said heat transmission to' said front surface and thus 'signaiing said injection and transmis- 2 L ,A, dsmwstratica a d. d sp a de s iunri e a r lat v ly his! su t z tiallyr ar su p t layer r id s; elati ely exhilarati n 'r a rfases; 'r a me ns om r ses.arlatalitys 1 s er to s te all srasqd p ss Qfsa' aw ei a e sss a r ac centrist: a .99. sai a lied ayer is s u s; fsa t yta are ever b 6,3!( la s! hei szt ns t th sash sa d th n sup axsra cprres ponding areas of said front surface; agd a yery thin thermosensitive layer applied ,.to.said frgntrsprface and comprising a thermosrisitiye ,substan pglor reversibly upon change inte r-ature i ndg dby nt surf n s naling the injection and transmission of heat into'and through said support layer.

:12. A demonstration and display device comprising: a relatively thin substantially planar support layer providing relatively extensive front and rear surfaces; layer means applied to said rear surface in selected portions thereof, said layer means comprising a plurality of mate-' rials differing in infrared radiation characteristics applied to adjacent zones of said rear surface, said applied layer means cooperating with an external source of energy to inject heat into said support layer in areas covered by said layer means, the resulting heat being transmitted through said thin support layer to heat corresponding areas of said front surface; and a very thin thermosensitive layer applied to said front surface and comprising a thermosensitive substance changing color reversibly upon change in temperature induced by said heat transmission to said front surface and thus signaling the injection and transmission of heat into and through said support layer, said source of energy being a source of infrahinge axis of said red radiation directing radiation toward one of said surfaces to produce zonal color changes in varyingjdegrees Within said thermosensitive layer "correspondingto' the different infrared radiation characteristics of said plurality of materials in said adjacent zones. i

""13. A demonstration and display device comprising: a relatively thinsupport layer providing relatively extensive front and rear "surfaces; layer means applied locally to said rear surface todefine a pattern thereon comprising at least one patterned layer of electricallyconductive material adapted to cooperate with an externalsource of electrical energy to be heated thereby, the heated material locally injecting heat into said support layer for transmission as a'heat pattern to said front surface, said rela- 'tively thin support layer being made of a materialtof low thermal conductivity to minimize -lateral' lieatconducti on beyond the edges of the pattern of said layer means; and a thermqse'nsitive layer'applied to said front surface and comprising a thermos'ensitive substance changing color reversibly upon change in temperatur in said'heat pattern asa result of changes in energy received by said material from said source; said theriiiosensi't ive substance being present at least in zones of said front substantially planar support layerp fov'ding relatively extensive front and reaf surfaces, layer means applied to said rear surface in selected portions thereofjl'said applied layer means cooperating with an external source df energylto inject heat into said supporting layer in areas covered by said layer means, theiesultiiig heat-being transmittedhhroughfi said thin support layer to heat corresponding areas of said-front surface, andayerythin thermosensiti've llayer' applied to said frontsurfac'e and comprising a thermosnsitive substance changing color revei's ibly upon change in temperature induced by said heat transmission to said front sprface and thus signaling the injection and transmission of 'heat into through said support layer one of said pages including a cutdut page portion hinged thereto along'aline spaced from :the pages, there 'being at' least :on e ar m hinged at one siesta the adjoining page al ong a ine spaced from saidzhinge axisand hirfgedjat its other-side to saidcutout page portion to swing th elatter intoa plane substantially parallel to-but spaced the plane of said adjoiningpageas said pages are syvung fro folded to unfolded'position, i i

15; A demonstpation and display deviee as defined in claim -14 in whigh saidcutout pag e pgrtion presents a pattern and 'is dispo s'ed to cast a corresponding shadow pattern on its substantially parallel page when the cutout page portion is between a source of infrared radiation and such substantially parallel page.

16. A demonstration and display device comprising: a relatively thin substantially planar support layer providing relatively extensive front and rear surfaces; layer means applied to said rear surface in selected portions thereof comprising an electrically conductive resistance layer adapted for connection to a source of electric energy and disposed to heat substantially uniformly an area of said support layer, the resulting heat being transmitted through said thin support layer to heat corresponding areas of said front surface; and a very thin thermosensitive layer applied as a pattern to said front surface in said area of said support layer and comprising a thermosensitive substance changing color reversibly upon change in temperature induced by said heat transmission to said front surface and thus signaling the injection and transmission of heat into and through said support layer. 17. A demonstration and display device as defined in tion.

18. A demonstration and display device comprising: 1

a relatively thin support layer providing relatively extensive front and rear surfaces; a first layer means applied 1 to one of said surfaces and cooperating with an external source of energy to inject heat into said support layer in areas covered by said first layer means, the resulting heat being transmitted through said support layer to heat corresponding areas of the other of said surfaces; a second layer means applied as a thin layer to said other surface comprising a thermosensitive substance changing color reversibly upon change in temperature induced by said heat transmission to said other surface and thus signal- I ing said injection and transmission of heat into and through said support layer, said thermosensitive substance having a reflectance characteristic curve characterized by a portion of high reflectance level in a longer wavelength range and a portion of low reflectance level in a shorter wavelength range, said portions being joined by a steep portion intermediate said high-level and low-level portions in an intermediate wavelength range, said thermosensitive substance being susceptible to photochemical color changes by receipt of radiation of wavelengths in said shorter wavelength range from ambient light; and a protective filter means associated with said device and positioned adjoining said second layer means, said filter means being relatively absorptive in said shorter wave, length range but transparent in said intermediate wavelength range and in the adjacent portion of the longer wavelength range.

19. A demonstration and display device as defined in claim 18 in which said filter means has a transmission characteristic substantially matching said reflection characteristic of said thermosensitive substance.

20. A demonstration and display device comprising: a relatively thin support layer providing relatively extensive front and rear surfaces; a first layer means applied to one of said surfaces and cooperating with an external source of energy to inject heat into said support layer in areas covered by said first layer means, the resulting heat being transmitted through said support layer to heat corresponding areas of the other of said surfaces; a second layer means applied as a thin layer to said other surface comprising a thermosensitive substance changing color reversibly upon change in temperature induced by said heat transmission to said other surface and thus signaling said injection and transmission of heat into and through said support layer, said thermosensitive substance comprising the double iodide of mercury and silver exhibiting a temperature-induced color change characterized by a change in absorption in a wavelength region above about 500 millimicrons and susceptible to graying by exposure to ambient wavelengths shorter than about 500 millimicrons; and a protective filter means associated with said device in a position superimposed relative to said thermosensitive substance, said filter means being relatively absorptive of said shorter wavelengths and relatively transparent to said longer wavelengths.- 1 e 21. A demonstration and display device comprising a relatively thin support layer means providing relatively extensive front and rear surfaces; a first layer means applied to one of said surfaces and cooperating with. an external source of energy to inject heat into said support layer means in areas covered by said first layer means, the resulting heat being transmitted through said support layer means to heat corresponding areas of the other of said surfaces; and second layer means applied as a thin layer to said other surface comprising a thermosensitive material changing color reversibly upon change in temperature induced by said heat transmission to said other surface and thus signaling said injection and transmission of heat into and through said support layer means, said thermosensitive substance having a reflectance characteristic curve characterized by a portion of high reflectance level in a longer wavelength range and a portion of low reflectance level in a longer wavelength range and 'a portion of low reflectance level in a shorter wavelength range, said portions being joined by a steep portion intermediate said high-level and low-level portions in an intermediate wavelength range, said thermosensitive substance being susceptible to photochemical color changes by receipt of radiation of wavelengths in said shorter wavelength range from ambient light, at least one of said layer means including a small amount of a humidifying agent to attract moisture and thereby aid in preventing said photochemical color changes.

References Cited in the file of this patent UNITED STATES PATENTS 713,643 Jackson Nov. 18, 1902 737,281 Sachs Aug. 25, 1903 1,457,209 Chanier May 29, 1923 1,591,029 Feldkamp July 6, 1926 1,693,369 Cochran Nov. 27, 1928 1,708,504 Hunt Apr. 9, 1929 1,857,019 Hassell et a1. May 3, 1932 1,975,052 Roos Sept. 25, 1934 2,155,114 Black Apr. 18, 1939 2,160,907 Richardson June 6, 1939 2,172,229 Waldo Sept. 5, 1939 2,229,828- Wach Jan. 28, 1941 2,248,604 Boersch July 8, 1941 2,308,087 Lappola Jan. 12, 1943 2,523,339 Sprecher Sept. 26, 1950 2,642,538 Urbach June 16, 1953 2,740,896 Miller Apr. 3, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,945,305 July 19, 1960 Allen Strickler It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 39, for "other surfaces" read other surface column 7, line 47, for "letters" read letters column 14, line 50 for "constuction" read construction column 14, line 51, before "Figa 26" insert of--; column 16,, lim

25, for "fixed" read fixedly column 20, lines 24 and 25, strike out "longer wavelength range and a portion of low reflectanct level in a".

Signed and sealed this 27th day of December 1960.

(SEAL) Attest:

KARL H. AXLINE ROBEl iT c. WATSUN Attesting Officer Commissioner of Patents column 14, line 51, before "Fig, 26" insert of; column 16, line UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,945,305 July 19, 1960 Allen Strickler It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 39, for "other surfaces" read other surface column 7 line 47, for "letters" read letters column 14, line 50 for "constuction" read construction 25, for "fixed" read fixedly column 20 lines 24 and 25, strike out "longer wavelength range and a portion of low reflectance level in a".

Signed and sealed this 27th day of December 1960.

(SEAL) I Attest:

KARL AXLINE ROBERT c. wAT-soN Attesting Officer Commissioner of Patents 

